The answer is 2
Step-by-step explanation: 100% is the highest score you can have . then you have to divide 25 by 100 because 25 is the total amount of questions ,you should get 4,wich means each question is worth 4 points.
18 correct is a score of 72
20 correct is a score of 80
4x2=8 wich you need 8 more to get to 80 from 72
feel free to ask for more help :)